Platoon commander Etienne Fabassou was not surprised by the Boko Haram ambush. Just a few days earlier, one of his colleagues had found a bullet in an envelope outside the Cameroonian military outpost at Zelevet where he is based, less than half a kilometre from the border with Nigeria.

Every night the dogs bark as intruders creep around the base, flashing torches. Boko Haram want the Cameroonian soldiers to know that they are being watched. Last week villagers spotted men planting homemade landmines along a dirt road, then disappearing back into the mountains on the other side.

“We are constantly under threat,” said Fabassou. “It’s always like this.”

He was speaking from a small military hospital in Maroua, the capital of Cameroon’s Far North region, which has been plagued by Boko Haram attacks for the last two years. In last Saturday’s ambush, a bullet went through his right arm. Sitting up in bed, dressed in a faded desert camouflage T-shirt, he completed his log of the incident, in which two Cameroonian soldiers were killed and three injured.

“They opened up on us,” said Fabassou. “I heard the shots and went down to look. We were overwhelmed by their firepower. They were hiding in the rocks. We didn’t see them because they were shooting from the caves.”

The Cameroonian army claims it killed dozens of jihadis at Zelevet but the truth is that it does not know, as it cannot go in hot pursuit over the border. More than 24 hours after the battle began, smoke from Cameroonian mortars was still rising from the mountains on the Nigerian side.

Women in the village of Bia, which was attacked by Boko Haram. Photograph: Soren Munk

“I have no faith in Nigeria – neither the government nor the army,” said a senior Cameroonian officer, who did not want to be named. “If things are calmer it’s not because of what the Nigerian army has done but because the Chadian forces have absorbed the intensity of the fight.”

Inside Nigeria's Sambisa forest, the Boko Haram hideout where kidnapped school girls are believed to be held

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Over the last month, the Nigerian army, reportedly backed by white mercenaries from South Africa, has started to flush fighters out of the Sambisa forest. In the long term, this should further weaken Boko Haram but the immediate impact is to drive its fighters into the Mandara mountains on the border with Cameroon. In the last few weeks, cross-border incursions have increased as the jihadis raid for food and women.

“We don’t understand why the rest of the world hasn’t helped us more,” said Colonel Jacob Kodji, the regional commander. “We need help from developed countries. You can see everything from your satellites, but we can’t place a soldier every few metres along a 400km-long border.”

Cameroon’s Israeli-trained Rapid Reaction Forces, known by their French acronym BIR, are the country’s most effective fighting force. Armed with American and Israeli assault and sniper rifles, 50mm heavy machine guns, 105mm cannon and mortars, they have succeeded in deterring many attacks, but are finding “hearts and minds” less easy to capture.

They mount regular patrols in the village of Bia, which was attacked by Boko Haram on 17 April. Ten people were killed, 30 houses burned and three girls kidnapped. The intruders also stole scores of cattle. The village elders, wearing pale silky robes and embroidered hats, lined the route as armoured personnel carriers and jeeps carrying BIR soldiers entered. The women, dressed in luminescent wraps, sat on the ground near the village centre.

“We were sleeping when Boko Haram came,” said Yabundi, a young woman in a red and yellow sari-type cloth. “It was midnight. Suddenly we heard gunfire so we woke up. Mothers gathered up their babies and ran; so did the men. They set fire to houses and all our belongings were burnt. We have nothing left, absolutely nothing.”

The people of Bia are from the Kanuri tribe, which straddles the border and provides the majority of Boko Haram fighters. Here, the conflict is more like a classic African bush war than a battle against Isis, to which the Boko Haram leadership declared allegiance in March. Some boys from Bia have joined Boko Haram, and the villagers feel caught between two sides.

“The soldiers said there were Boko Haram in this village, but we said no,” explained an elder called Modi. “Then they said if you catch one you must hand him over to us, and if you refuse we’ll do exactly the same to you as we would to him. The military threatened us.”

Walking through the village, the men point out the houses that were burn ed – all, they say, belonging to families who refused to co-operate with Boko Haram.

“A son of the village who had earlier joined Boko Haram appeared in the village and our former chief called the military who took the boy away,” explained Modi. “So then Boko Haram accused us of helping the military and they came and launched this revenge attack on us.”

To the soldiers, the people of Bia are suspects as much as citizens in need of protection.

“They’re brothers and sisters, children, all people of the same family,” said Lieutenant Yari Emmanuel, a local Rapid Reaction Force commander. “It’s not easy for someone to give up a brother or sister – even if he or she is the devil.”

In the last two weeks, 2,000 new refugees from Nigeria have arrived at Cameroon’s largest camp, Minawao, joining 35,000 who have been there for a year or more. The old refugees are suspicious of those who delayed fleeing until now, especially as many of the new arrivals, who are mostly women, are vague about the whereabouts of their husbands and appear to have wiped their sim cards.

“If you stayed back until now, while Boko Haram are there, yet they did not kill you, it means you are working with them,” said Isaac Luka, the camp president, who fled in June last year. “Now the military in Nigeria is working hard, these people are afraid to stay because the military may not leave them alone. That’s why they’re running now.”

General Muhammadu Buhari, who assumes the Nigerian presidency at the end of this month, has promised his neighbours that he will do more to crush Boko Haram. Until he does, there is little chance that the Far North of Cameroon can return to stability.

Lindsey Hilsum is international editor for Channel 4 News. Her report can be found at www.channel4.com/news